The invention concerns a method and a device for process control using Fourier transform (FT) infrared (IR) spectroscopy in which various substances are fed into a storage vessel and stirred.
Measurements in FT IR spectroscopy assume and require that the IR radiation being analyzed is static (constant in time) during the period of time during which the measurement is carried out. An interferogram is generated typically by driving a mirror in a forward and backward direction at substantially constant velocity to generate an intensity versus mirror position or path exhibiting a so-called central burst containing the spectral information. A plurality of measurements are normally carried out and added together to improve signal to noise ratio. This procedure requires a fixed phase relationship between the interferograms which are added together. Subsequent to addition, the spectrum is extracted via Fourier transformation.
Application of this procedure to analytic measurements is not possible if there are time dependent, dynamic changes in the interferogram caused by disruption of the measurement system (the sample, the detector, the electronics, etc.) during the measurement time. In applications involving chemical production processes, liquids and gases are added to a production vessel, are generated during the production process, and/or are stirred and mixed. Such processes require periodic changes in system parameters such as temperature, the gas additive constituents, the liquid additives etc. Dynamic feedback control and regulation of such systems is required in order to check whether or not acceptable conditions have been established prior to carrying out the next step.
Use of Fourier transform infrared spectroscopy for chemical process control is difficult, since the stirring and mixing processes cause changes in the physical properties of the sample during the measurement. These dynamic processes are associated with random disturbances of various character which cause major disruptions in the recorded interferogram which distorts the subsequently Fourier transformed spectrum. These distortions can be of such a substantial magnitude as to preclude analysis of the chemical composition of the reactants using Fourier transform IR spectroscopy.
In view of these problems, it is the underlying purpose of the invention to provide a method and an apparatus which is capable of recognizing and compensating for dynamic disturbances in process controlled chemical processes such that analysis using Fourier transform infrared spectroscopy is possible.
This purpose is achieved in accordance with the invention by passing a beam of infrared radiation through a chemical composition, generating an interferogram of the infrared radiation, analyzing regions of the interferogram for intensity fluctuations, labeling the interferogram as being acceptable or non-acceptable in dependence on the analyzed intensity fluctuations and adding together the acceptable interferograms to form a sum interferogram, prior to Fourier transformation.
In accordance with the invention, it has been discovered that the major source of disturbance for IR FT applications in such chemical processing control methods is caused by the formation of bubbles passing through the liquid samples. These bubbles result in widely varied disturbances in the measured interferograms in dependence on the size, speed and direction of travel of the bubbles through the measuring region. Studies of bubble formation in these chemical processes have led to the realization that the bubbles have a characteristic frequency, time and amplitude dependence in the interferograms. Through inspection of the portions of the interferograms specific measurements can be analyzed and selected, wherein interferograms containing substantial bubble interferences can be reliably eliminated from the spectrum. The inspected regions can be integrated, differentiated or be subjected to various other mathematical operations.
In a preferred embodiment of the invention, the selection process for acceptable interferograms comprises analysis of either the amplitude, the time, and/or the frequency dependence of the intensity fluctuations. This embodiment has the advantage that a characteristic signature of bubble formation in the measurement can be analyzed and extracted in order to determine which interferograms can be included in the sum spectrum and which must be discarded.
In an improvement of this embodiment, the intensity of fluctuations within an analyzed region are added together to generate a sum value. This improvement has the advantage of reducing investigation of disturbances contained in the interferogram to one single parameter. After establishment of an acceptable value for this parameter, the parameter can be used as a signature for acceptable and non-acceptable interferograms.
In a preferred application of this improvement, the sum value is compared to a maximum allowed value. This aspect of the invention involves realization that the intensity changes in the non-spectral region of the interferogram are subject to fluctuations about a mean value in both positive and negative directions. Analysis of the maximum possible positive excursion of the sum value has shown, in accordance with the invention, that a single parameter can be extracted from the interferogram characteristic of acceptable and unacceptable levels of bubble formation.
In another aspect of this improvement, the sum value is compared to a maximum possible negative value. This aspect takes into consideration the fact that the average positive and negative excursions of the intensity fluctuations from a zero value do not, in general, average out to precisely zero over a region investigated. It has turned out that the degree of deviation of the average value from a zero value is a measure of the degree of disturbance in a given measurement. This parameter is a suitable one for selecting acceptable and non-acceptable interferograms.
In an improved embodiment, standard commercially available spectroscopy software procedures are used to select acceptable interferograms. This embodiment has the advantage that the user does not have to write his own software algorithms in order to analyze regions of the interferogram in order to decide whether or not a given interferogram should be selected. Instead, standard analysis programs available to the user, e.g. for summing regions of spectra, for analyzing the frequency dependence of regions of spectra etc., can be utilized to generate the criteria for acceptable interferograms.
The invention is also directed to a device for process control of chemical processes using Fourier transform infrared spectroscopy. The device involves means for feeding compositions into a reaction vessel and means for stirring the composition. In accordance with the invention, means are provided for passing a beam of infrared radiation through the composition and for measuring an interferogram of infrared radiation which has passed through the composition. Analysis means are provided for inspecting regions of the interferogram for intensity fluctuations. Means are provided for labeling interferograms having fluctuation disturbances below and/or above a certain level, wherein spectra are added together to form a sum interferogram comprising only those spectra selected for acceptable interference levels.
Various embodiments of this device provide means for carrying out the various method steps discussed above and will not be described in further detail here.
Further aspects of the invention are delineated below in a preferred embodiment with reference to the drawings. The particular features disclosed in the drawing and in the specification as well as the claims can be important to the invention individually or collectively in arbitrary combination. The embodiment is not an exhaustive enumeration of inventive configurations rather has exemplary character only for illustration of the invention and its best mode.